In Condition:
* Modify constructor to accepts tests and a default return value;
* Implement #met?;
* Modify #! so that it return the negated value returned by #met?.
In Condition::DSL:
* Modify .evaluate to build the condition with tests and the value
returned by a the evaluated condition block.
Add a basic Test base class, with env and arguments as attributes.
Add some spec helpers to build some easily testable kind of Test
instances (as test doubles).
* Throw symbols instead of raising exceptions to test code evaluation,
this will reduce "bad" usages of exceptions, symbols seems a little
better for now;
* Add a specific exception to be used in specs for testing behaviour
when an exception is raised from a recipe.
